Protein-wise, an all cabbage diet would give you more if you’re meeting calorie needs - 5.1g protein per 100 calories, or 102g protein for 2000 calories worth of cabbage.. but that is a heck of a lot of cabbage (17ish lbs)!
Let’s be real though, people should be eating a varied diet and not just a single food. And perhaps not a junk food only diet.
When I was growing up, there was a vogue among my fellow vegetarians for the book Diet for a Small Planet which suggested that we needed to eat a diversity of amino acids in each meal, hence "complementary" proteins at the same time. This concept then seemed to fade away completely because it appears that the body can actually successfully make use of amino acids even when consumed at different times. But they have to be consumed eventually!